1. Field of the Invention
This invention relates in general to a text editing and display system having input devices such as keyboards, for entering textual characters into the system, and a central processor unit and a memory unit for receiving and storing the display characters for subsequent display by a display unit, such as a cathode ray tube raster scan display. More specifically, it relates to such a system having the capability to insert display characters into the text stored in memory and thus also into the text displayed by an associated display unit.
2. Background
There are a number of text editing and display systems which, typically, include keyboard input terminals, cathode ray tube (CRT) display terminals, central processing units for controlling the systems and memory units for storing the display characters. An operator at each keyboard can enter display characters into such a system. Characters thus entered are stored in the memory unit by the processing unit and are retrieved from the memory unit by a control unit, which may be the processing unit, for display on the CRT screen. Such a system usually has the ability to insert display characters into the text stored in the memory unit.
For normal data entry, the keyboard operator positions a display cursor and "types" in the selected display character at the cursor location. The entered character "strikes over," or replaces, any character in that location. The operator initiates a text insert operation by positioning the cursor at the location of the desired insert and depressing a momentary or a locking "insert" key which generates an insert signal. This key may be mechanically identical to the "SHIFT" key used on standard office typewriters.
In prior systems, the terminal is thus enabled to transmit an insert signal to the central processing unit. In response, the central processing unit returns an acknowledgment signal to the keyboard which sets a latch to actuate a display device, such as a light, indicating to the operator that the system is now in the insert mode of operation. The operator then types one or more characters and, as each character is entered, the character under the cursor and all subsequent characters are moved to the right or to a subsequent line. One problem with systems of this type is that line noise between the processor and input terminal may erroneously establish or terminate an insert mode indication at the keyboard.
Thus, these prior character insert systems require that an "insert mode" be maintained between the processor and the keyboard terminal while the inserted characters are being entered. Consequently, the processor is required to check a "mode table" each time it receives a character to determine if the character is to be inserted or added normally into the text. Thus, the processor must perform an additional operation each time a character is entered, which complicates the software controlling the processor and increases processing time. In addition, bidirectional data must be transferred over the lines between the keyboard terminals and the central processor unit, adding a wire between the central processing unit and input terminal and causing further complexity at the terminal (e.g., command decoding circuitry).
It is an object of this invention to provide a text editing and display system which efficiently uses processor time.
Another object of the invention is to insert display characters into a stored text without establishing an insert mode between the remote keyboard and the central processor.
A further object of this invention is to insert display characters into previously stored text without requiring two-way communication between the central processing unit and the data terminal.
It is still a further object of this invention to provide a text editing and display system having an insert capability which is resistant to line noise problems.